PHYSIOLOGY OF THE SPERMATOZOON 99 



fixed to it at once or, more often, they rotate upon it; 

 and in the latter case, looking from them to the surface 

 in question, in a counterclockwise direction." 



In contact with any solid object Nereis spermatozoa 

 tend to carry out circus movements in an anticlockwise 

 direction when fresh, but may soon come to rest. This 

 thigmotactic reaction appears to be due to exaggeration 

 of the rotation component of the ordinary spiral course; 

 it is the cause of aggregations in favoring places, such 

 as angles, etc. Such contact reactions of spermatozoa 

 must be regarded as very significant for fertilization. 



The change from a progressive to a rotary form of 

 locomotion thus appears to be a very general form of 

 reaction of spermatozoa to contact stimuli. This may 

 take place without any change in the rate of activity 

 or of metabolism and would obviously lead to aggre- 

 gation of spermatozoa in any region providing a stimulus 

 which acts in this way. 

 3. In Relation to Reaction of the Medium; H and OH Ions 



a) Activity effects. The spermatozoa of many ma- 

 rine organisms are extremely sensitive to changes in 

 the normal reaction of the sea-water, as indicated by 

 their activity. In the case of Nereis the addition of 

 acids (sulphuric, hydrochloric, nitric, acetic) to the 

 sea- water caused paralysis at w/i,ooo and decrease of 

 activity up to n/ 5,000. The spermatozoa of sea urchins 

 are not so sensitive and those of some other marine 

 forms much less so. If sea- water is saturated with 

 CO 2 the spermatozoa of all forms are completely para- 

 lyzed, and this paralysis will last in the case of Nereis 

 until the charged sea-water is diluted over a hundred 

 times; in i per cent of the CO 2 charged sea-water the 



